I based the size of my koozie on a beer bottle, but there is about two inches of overlap, so if you want to make yours more versatile, you can place two rows of buttons so that you can use it for a bottle, can or as a coffee koozie.
Here is how i did it:
DIY Drink Koozie
Insul-Bright fabric- cut in a 4.5 x 10 inch strip
2 pieces of coordinating fabric cut 4.5 x 10 inches
1 thin hair elastic (not the super thick one- the normal adult elastic)
Place koozie on the bottle/cup you want it to wrap around and position the buttons so that the elastic needs to stretch a little, but not too much to keep it closed. Sew the two buttons to line up with the elastics.